EVS-EN 61558-2-4:2001

This part 2 of IEC 61558 applies to stationary or portable, single-phase or polyphase, air-cooled isolating transformers, associated or otherwise, having a rated supply voltage not exceeding 1000 V a.c. and rated
frequency not exceeding 500 Hz, the rated output not exceeding: - 25 kVA for single-phase transformers;
- 40 kVA for polyphase transformers. This standard is also applicable to isolating transformers without limitation of the rated output, however such transformers are considered as special transformers and are subject to an agreement between the purchaser and the supplier. The no-load output voltage or the rated output voltage does not exceed 500 V a.c. or 708 V ripple-free d.c. The no-load output voltage and/or the rated output voltage shall exceed  50 V a.c. and/or 120 V ripple-free d.c.